1. Field of the Invention
The invention relates to aircraft pitot probes, and particularly to controllers for regulating the pitot probe temperature.
2. Description of the Prior Art
A pitot probe is mounted outside the aircraft below the cockpit. These probes vent the total air pressure induced by the forward motion of the aircraft into the aircraft pressure vessel whereat the pressure is sensed by the air data system. Since the pitot probe is exposed to aircraft's exterior environment, it must be heated to maintain the pressure inlet free of ice. When the aircraft systems are powered up, full power is applied to the pitot probe heater. If the temperature is not controlled it may be in the range between 800.degree. F. and 1100.degree. F. when the aircraft is not flying. Such excessive temperature causes many pitot probes to prematurely fail.
To prevent premature failure the prior art employs a "Half Power" switch which couples only half the in-flight applied power to the pitot probe when the aircraft systems are powered up and the aircraft is on the ground. Generally this is accomplished by half wave rectifying the 115 V ac heater voltage. The switch is held in the "Half Power" position by an on-ground signal, as for example a weight on wheels signal, which is applied when the aircraft is on the ground. When the on-ground signal indicates that the aircraft has left the ground, the switch is activated to remove the half wave rectification and full power is applied to the pitot probe heater element. The half wave rectification reduces the probe temperature on ground to the range between 500.degree. F. and 800.degree. F. from the full power range between 800.degree. F. and 1100.degree. F. Though a significant temperature reduction is realized, many probes protected by the "Half Power" switch continue to experience high temperature related failures.